MAMMALS OF THE MEXICAN BOUNDARY. 



363 



KEY TO TEX GENERA OF MURID.E.O 



a. Grinding teeth crowned by tubercles or cusps. 



b. Grinding teeth with tubercles arranged in three transverse rows, very distinct in teeth 



of upper jaw Mvs (p. 363). 



hh. Grinding teeth with tubercles arranged in two longitudinal rows. 



r. Upper incisors longitudinally grooved Reithrodontonujs (p. 450). 



cr. Upper incisors not grooved. 



(/. Skull with a distinct ridge over eye-socket; fur coarse; Ix-liy not pure white; size 



larger (total length usually over 130 mm) Oryzomys (p. 454). 



d(l. Skull without a prominent ridge over eye-socket: hcily jjure white; smaller 

 (total length usually vmder 130 mm). 

 e. Coronoid process of mandible developed as a long hook; .soles of feet haiiy. 

 /. Body stout; size medium to rather large (total length 125 to 175 mm.); 

 plantar tubercles, four; teeth with liigh iwhevcXes. . . .Onychomys {\). 3G8). 

 ff. Body mouse-like; size small (total length less than 100 mm.); plantar tuber- 

 cles, six; teeth with low tubercles Baiomys (p. 381). 



ee. Coronoid process of mandible developed as a short spine ; soles of feet largely 



naked Peromyscns (p. 382). 



aa. Crowns of grinding teeth flattened, and divided into loops or triangles formed hy plates 

 of enamel inclosing dentine. 



g. Enamel loops S-.shaped Sigmochm (p. 445). 



gg. Enamel loops distinctly triangular. 



Ji. Upper incisors narrow, compressed, the antero-posterior diameter of each 

 much greater than the transverse diameter; body slender; tail always 



long: eyes and ears large; belly white Neotoma (p. 467). 



hh. Upper incisors broad, the antero-posterior diameter of each less than 

 transverse diameter; body clums)'; tail usually short; eyes and ears 

 small; belly generally not white. 



i. Tail flattened laterally Fiher (p. 494). 



ii. Tail round Mlcrotns (p. 498) 



Sulafkinily IVtXJRIIsr^E;. 



Molars rooted and tuberciilated, those 

 of the upper jaw with three longitadinal 

 rows of tubercles. The species existing 

 in the New World have been introduced 

 and naturalized from the Old World. iM^^^ 



^.^^ ('^^^\ 



Genus MUS Linnaeus (1758). W A^^S "^^^^ 



Dentition—I. t^| ; M. H = 16. ^^P^ ^ 



o be 



Front teeth two; cheek teeth never figss.-mus .vlexandrinus, 

 more than three in each iaw. Incisors "• skull; 6, ltper molars; 



n e. Lower mol.\rs. 



narrow, without grooves. Structure or 



molars as in fig. 58. Incisive foramina of skull long; coronoid proc- 

 ess of mandible well developed. Ears and eyes rather large. Muzzle 

 naked at the extremity. Fur soft, in some cases mixed with spines. 



a Largely borrowed from Miller's Ke}^ to the Land Mammals of Northeastern North 

 America, in Bulletin of the New York State Museum, VIII, October, 1900, p. 93. 



